1. Field of the Invention
The present invention relates to an apparatus for combining and projecting a plurality of images through light transmission type image panels, such as liquid crystal panels, and also relates to a method and an apparatus for adjusting the image combining and projecting apparatus.
2. Description of Related Art
In a known three-plate type of liquid crystal color projector, white light emitted from a light source is separated into three primary colors of red (R), green (G) and blue (B) by a dichroic mirror, so t hat the three colors are made incident upon three respective liquid crystal panels which indicate red, green, and blue image data. The three beams of colored light are projected onto a screen through an image combining means, such as condenser lenses corresponding to the liquid crystal panels, and a projection lens to obtain a color image. In an apparatus that employs this type of optical arrangement, the condenser lenses are provided in front (i.e., optically upstream) of the liquid crystal panels.
As is well known, transmittance of a beam incident upon a liquid crystal panel depends on the incident angle of the beam. Namely, the liquid crystal panel has an irregular transmittance that is dependent upon the angle of beams incident thereon.
Consequently, in the known optical arrangement as mentioned above, in which the condenser lenses are located on the optically upstream side of the liquid crystal panels, the beams converged by the condenser lenses are made incident upon the respective liquid crystal panels. Accordingly, the image projected on the screen may be adversely affected by an irregularity in the transmittance due to a difference in the incident angle.
To eliminate the adverse influence, recent image combining and projecting apparatuses have condenser lenses that are located behind the liquid crystal panels, i.e., on the emission side of the liquid crystal panels. In this arrangement, the collimated beams (parallel beams) are incident upon the liquid crystal panels and the beams emitted from the liquid crystal panels are condensed by the condenser lenses
Nevertheless, the improved arrangement of the condenser lenses behind the liquid crystal panels has invited another problem as discussed below. Namely, in a three-plate type of liquid crystal projector, it is necessary to make three images, formed by the three liquid crystal panels, precisely coincident with each other to obtain a clear color image. Arrangement of the condenser lenses on the outgoing side of the liquid crystal panels results in the possibility of distortion when the optical axes of the condenser lenses are deviated from the optical axis of the projection lens. The distortion that results under such circumstances is asymmetric. Therefore, if the optical axes of the condenser lenses corresponding to the three liquid crystal panels vary with respect to the optical axis of the projection lens, a difference in shape of the projected images formed by the respective liquid crystal panels will result. The projected images being deviated from one another will result in a dimmed or unclear color image.
If, in addition to trapezoidal distortion, a mounting error occurs upon assembly of the three liquid crystal panels, the corresponding condenser lenses, and/or the projection lens which combines and projects the three colored images, etc., three colored trapezoidal distortions, which do not correctly overlap, are projected onto the screen. To eliminate the trapezoidal distortions which do not correctly overlap, it is necessary to precisely position the above-mentioned elements of the assembly. However, this requires a fine and troublesome adjustment of the assembly.
On the other hand, if the condenser lenses are arranged on the incident side of the liquid crystal panels, the deviation of the optical axes of the condenser lenses from the optical axis of the projection lens are theoretically the main cause of irregular illuminance. However, the trapezoidal distortion resulting from the deviation of the optical axes of the condenser lenses is not as significant in this arrangement. Generally speaking, there is a larger allowance for the deviation of the optical axes of the condenser lenses from the optical axis of the projection lens when the condenser lenses are arranged on the incident side of the liquid crystal panel than when the condenser lenses are arranged on the outgoing side of the liquid crystal panels. Therefore, positional adjustment of the condenser lenses has not been taken into consideration in the known arrangement in which the condenser lenses are located on the incident side of the liquid crystal panels.
In the known liquid crystal color projector as mentioned above, light combining means, such as dichroic mirrors or dichroic prisms are inserted between the projection lens and the liquid crystal panels to combine images formed by the liquid crystal images and thereby obtain a projection image. However, the dichroic mirrors or the dichroic prisms have an inevitable manufacturing error or mounting error (assembling error), etc., which results in a different position for each of the three colors.
There is another problem in the case that a zoom lens is used as the projection lens. In a zoom lens, as is well known, it is necessary to effect a zoom adjustment in which the focal position is kept constant, even if the zooming is carried out by the zoom lens system, and a back focus adjustment (back distance adjustment) in which the position of a liquid crystal panel is made coincident with the focal position obtained by the zoom adjustment.
For example, in the case of a simple zoom lens having two groups of lenses, generally speaking, the zoom adjustment is effected by the movement of the first lens group in the optical axis direction, and the back focal adjustment is effected by the axial movement of the first and second lens groups in combination. However, in a projector in which a plurality of images are combined and projected, the focal point of the three colors varies due to errors which inevitably occur in the manufacturing process or when mounting (assembling) the dichroic mirrors or the dichroic prisms, etc., as mentioned above. Accordingly, it is impossible to project all the color images with a correct focal point, even if the zoom adjustment and the back focus adjustment are carried out by the zoom lens in the known focal adjustment method, as mentioned above.
Furthermore, in the known liquid crystal color projector as mentioned above, it is preferable that the optical axis of the beam separating optical system, which separates the beams emitted from the light source and makes them incident on the liquid crystal panels, is coincident with or at least parallel to the optical axis of the beam combining and projecting optical system for combining the beams transmitted through the liquid crystal panels. However, due to the possibility of a mounting error and irregularity in the position of the light source and the various mirrors, etc., a slight positional deviation between the axis of the beam separating optical system and the axis of the optical axis of the beam combining optical system may result. Generally speaking, these axes intersect at an angle. Consequently, only a part of the beams of the beam separating optical system can be received by the projection lens, resulting in an irregular amount of light which is received and an uneven color distribution, etc. In order to effectively gather all the beams transmitted through the beam separating optical system, it is necessary to use a bright projection lens having a large beam receiving angle, i.e., a small F-number. However, this is very costly.